King v Western Sydney Local Health Network [2013] NSWCA 162

The recent decision of King v Western Sydney Local Health Network1 from the New South Wales Court of Appeal confirms the approach taken by the High Court decision of Amaca Pty Ltd v Ellis2 to the way in which epidemiological studies can be applied in medical negligence cases to prove factual causation; focussing in particular on the distinction between probability and mere possibility in relation to a material increase in risk.

Here, Hoeben and Ward JJ (with Basten JA dissenting) rejected the appeal, and held that the evidence relied upon by the appellant, including epidemiological evidence, established at best, proof of a possible material increase in risk, and therefore was insufficient to establish factual causation on the balance of probabilities.

Material Facts

On 6 May 2002, Mrs King, who was three months pregnant with the appellant Tamara, presented to the Blacktown Hospital within 24 hours of her eldest daughter Shania developing symptoms of chicken pox (varicella). Despite instructing the Hospital that she did not believe she had immunity to varicella, Mrs King was not given an injection of Varicella-Zoster Immunoglobulin (VZIG), but rather was advised to undertake a blood test and asked to return in three days for the results at her next antenatal appointment. Mrs King returned as requested however there was no discussion of the blood test results, the immunisation or any potential risk to the appellant.

The feared harm came to pass. Mrs King subsequently developed varicella, and the appellant was born suffering from congenital varicella syndrome (CVS).

The appellant’s case against the respondent was that it had breached its duty of care by failing to advise Mrs King of the availability of VZIG, and offer VZIG to her on her attendance to the hospital on 6 May 2002, and as a result, had caused the appellant to suffer from CVS.

Judgment at first instance

In the judgment at first instance3, Garling J confirmed that the respondent’s duty owed to the appellant, which became enlivened upon her birth, consisted of an obligation to advise Mrs King of the availability of VZIG and its potential beneficial effects in preventing or ameliorating chicken pox and an obligation to have offered to administer VZIG to her.

His Honour found that the respondent had breached its duty, and the central factual causation issue was identified as being:

“Had, on 6 May 2002, the medical staff at Blacktown Hospital, appropriately advised about and administered 600 international units of VZIG to Mrs King, she would, on the balance of probabilities, not have contracted Varicella on 16 May 2002”.

The appellant provided epidemiological data, including a German study (referred to as the Enders table) showing the application of the drug across a group to establish that it was more probable than not that the application of that drug to Mrs King whilst pregnant would have prevented harm to the appellant.

Ultimately, Garling J concluded that the appellant had failed to establish the causal link between the administration of VZIG and the prevention of Mrs King contracting varicella. His Honour concluded that the Enders study was insufficient in that it established only the possibility that the varicella virus suffered by Mrs King could have been prevented, not the probability that it was more likely than not. He therefore held that general causation was the only conclusion open on the appellant’s evidence, stating:

“This comes about because the law regards proof as sufficient on the balance of probabilities. However, the law does not necessarily accept that without more, one can properly apply the results of such a population study to any one individual within a population. General causation, which is the only conclusion open on the Enders Table, whether adjusted or not, is only evidence of possibility and not probability”.4

The Court of Appeal Decision

The trial decision was appealed on the grounds that His Honour, inter alia, failed to give proper effect to the Enders table and should not have adjusted the data but simply relied upon evidence given, and further, had failed to apply the correct legal test to the question of causation.

Ultimately, Hoeben and Ward JA’s (with Basten JA dissenting) dismissed the appeal, holding that the appellant’s expert evidence failed to prove that had VZIG been administered to Mrs King, she would not have developed chickenpox. In particular, the appellant had failed to show how the general statistical information contained in the Enders table applied to the appellant's particular case.

Hoeben JA noted that in order for the appellant to successfully establish causation, the evidence of Professor Curtis, and in particular his reliance upon the Enders table, had to be accepted. The evidence of the other experts, not only did not provide support, but were to the contrary.5

His Honour referred to Spigelman CJ’s comments in Seltsam v McGuiness6:

In Australian law, the test of actual persuasion does not require epidemiological studies to reach the level of a relative risk of 2.0, even where that is the only evidence available to a court. Nevertheless, the closer the ratio approaches 2.0, the greater the significance that can be attached to the studies for the purposes of drawing an inference of causation in an individual case. The “strands in the cable” must be capable of bearing the weight of the ultimate inference.7

Further, he followed the reasoning set down by the High Court in Amaca Pty Ltd v Ellis 8:

The first step that must be taken, if an inference is to be drawn about causation from epidemiological studies, is to relate the results of studies of populations to the particular case at hand. That step is not inevitable.

Hoeben JA concluded that his Honour was mindful of the limitations of the Enders table and in particular, its applicability to Australia; it was by no means clear that the appellant constituted a “typical” member of the cohort on which the table was based. Further, he held that the trial judge’s reservations concerning the reliability of Enders table were justified because of its inherent weakness as an evidentiary basis, and found that the adjustments made by His Honour at first instance were appropriate in the circumstances.

With respect to the submission that the trial judge had failed to apply the correct legal test for causation, the appellant relied on the statement of principle by McHugh J in Chappel v Hart 9:

Before the defendant will be held responsible for the plaintiff's injury, the plaintiff must  prove that the defendant’s conduct materially contributed to the plaintiff suffering that injury. In the absence of a statute or undertaking to the contrary, therefore, it would seem logical to hold a person causally liable for a wrongful act or omission only when it increases the risk of injury to another person. If a wrongful act or omission results in an increased risk of injury to the plaintiff and that risk eventuates, the defendant’s conduct has materially contributed to the injury that the plaintiff suffers whether or not other factors also contributed to that injury occurring.

The appellant submitted that the hospital’s conduct in not offering to administer ZVIG to Mrs King increased the risk of injury to her, namely her mother contracting varicella, and therefore causation was established in that the hospital’s conduct materially contributed to the injury.10

Reference was also made to the analysis of material contribution in Strong v Woolworths Ltd 11, that is, proof of the causal link between an omission and an occurrence requires consideration of the probable course of events had the omission not occurred. Hoeben JA noted that case did not assist the appellant’s submission, but rather highlighted the significant difficulties in applying the “but for” test to the concept of “increase in risk”.12 He stated that the decision was:

“…strongly suggestive that in most of those cases, the “but for” test will not be satisfied and that recourse will have to be had to the “exceptional case” provision under section 5D(2)”13.

His Honour rejected the appellant’s submission, noting that to apply the Chappel v Hart test of causation in the way the appellant sought to do would always result in an affirmative result for a plaintiff in a case such as this, even where there was a very low prospect of infection being avoided by Mrs King. His Honour reasoned:

The infected person would always be able to say that the risk of infection had been increased by the failure to administer the therapeutic substance and therefore the risk had come home”. 14

Hoeben J noted that the issue, correctly stated, was whether an increased risk did cause or materially contribute to the injury actually suffered, concluding in line with previous authorities, that mere possibility was not sufficient to establish causation15.

In dismissing the appeal, Ward JA took a different approach to the issue of causation, noting that the central issue should not be whether administration of VZIG would have prevented Mrs King from contracting varicella, but rather, whether it would have prevented the appellant from contracting CVS.16

In applying the Enders table relied on by the appellant, Ward JA determined that the appellant’s chances of contracting FVS would have been ameliorated by less than 1% by the administration of VZIG to Mrs King, and therefore it was difficult to accept that the respondent’s breach caused or materially contributed to the applicant’s injury.17

Finally, it was noted in the judgement at first instance that the plaintiff did not rely on the “exceptional case” provision under section 5D(2) of the CLA18, however on appeal, counsel for the appellant sought to rely upon the exception by way of oral submissions. Ward JA commented that there may be an argument that this was an exceptional case where factual causation could not be proven, because of the ethical difficulties in conducting tests that would be more reliable in assessing the statistical link between administration of VZIG and the incidence of FVS. It is indeed worth noting Basten JA’s dissenting judgment on this point also, who believed the appellant would succeed on this point due to the lack of sufficient empirical evidence available for the plaintidfd to prove causation in fact.

The argument however, was not tested. The court concluded that the normative considerations for analysing the causation issues under the provision may require the adducing of evidence, and should therefore have been raised at the trial of the matter. Thus the court held that it was not appropriate for the Court of Appeal to be undertaking an enquiry as to the application of s5D(2) CLA in those circumstances.


Therefore, recovery for a plaintiff in cases for materialisation of risks will be limited to cases where the evidence can show for it to be probable, not just possible, that the plaintiff would have had a better outcome. This means, evidence put forward must show specific application to the plaintiff, not just a statistical likelihood in reference to the broader population.

Somewhat inadvertently, this case also highlights the potential benefit to plaintiffs of pleading exceptional circumstances in the alternative, in order to rely on the “exceptional case” provision contained in each state’s civil liability legislation in circumstances where factual causation is unable to be proven at trial.